Theiler's murine encephalomyelitis virus (TMEV)-induced demyelinating disease (TMEV-IDD) is widely considered the most relevant animal model of virus-induced autoimmune-mediated demyelinating disease. MS is believed to involve an autoimmune pathology, but epidemiological evidence strongly suggests a viral trigger. TMEV are natural mouse pathogens and intracerebral inoculation of susceptible SJL mice with the BeAn strain of TMEV results in a chronic-progressive, inflammatory immune-mediated demyelinating disorder which is related to life-long persistent CNS virus infection and characterized by spastic hind limb paralysis. In contrast, resistant C57BL/6 mice make a potent CTL response and rapidly clear the infection. Like MS, TMEV-IDD is characterized by progressive demyelination with accompanying mononuclear cell infiltrates dominated by CD4+ T cells and activated APCs (microglia/macrophages/DCs). Our previous studies have shown that demyelination is initiated by virus-specific CD4+ T cells targeting viral epitopes presented by persistently infected CNS- resident APCs. Chronic demyelination is mediated by induction of autoimmune responses to a variety of endogenous encephalitogenic myelin epitopes which arise via epitope spreading. This application proposes continue our productive studies funded by an NIH PPG for the past 21 years. Based on extensive new preliminary data, we will examine the role of innate immune regulatory and innate immune stimulatory mechanisms involved in regulating susceptibility/resistance to TMEV-IDD at both the level of immune- mediated virus clearance to acute infection and regulation of autoimmune processes during chronic demyelination. Employing depletion, supplementation and genetic approaches, Aim 1 will test the hypothesis that activation of CD4+CD25+Foxp3+ Tregs plays a major role in regulating susceptibility to TMEV-IDD by inhibiting development of virus-specific CD4, CD8, and antibody responses during acute infection leading to establishment of CNS virus persistence and hence eventual development of chronic autoimmunity. Aim 2 will test the hypothesis that the pro-inflammatory effects of both myelin epitope-specific autoreactive Th1 (IFN-3) and Th17 (IL-17) cells are critical for mediating demyelinating immunopathology during the chronic autoimmune phase of TMEV-IDD. Aim 3, will test the hypothesis that peripherally-derived, CNS-resident DCs play a critical role in driving the activation of epitope spreading to myelin-specific CD4+ Th1/17 cells responsible for CNS pathology in chronic TMEV-IDD. Innate immune responses (expression of cytokines; chemokines; and antigen presentation molecules); and functional ability of CNS-derived DCs, MUs and microglia from TMEV-infected mice to activate T cell proliferation and differentiation of naove CD4 and memory CD4+ (Th1, Th2 and Th17) and CD8+ T cells by will be determined. These studies will provide important information on the mechanisms underlying susceptibility/resistance in virus-induced demyelination and are applicable for the future design of treatment strategies for MS and other CNS inflammatory diseases.